Add docs for mc-stan.org

# Feature request

I am adding this issue to let people know that I am starting work on importing documentation for mc-stan.org. I had previously proposed this documentation on the gitr channel and got positive feedback for the idea.

## Summary

The mc-stan documentation is somewhat similar to the scikit-learn documentation, and it is split into several parts:

1. User's guide
2. Stan language reference
3. Stan functions reference
4. User guides for projects building on top of Stan
5. Case studies
6. Tutorials

I think points 1.-3. could be added to devdocs.io. I think point 3. is lowest hanging fruit, so I will start with that.

The documentation for 1-3 is located at https://github.com/stan-dev/docs

首先查看 https://github.com/stan-dev/docs 中的 mc-stan 文档仓库，先重点关注被确定为最容易着手的 Stan 函数参考。确定文档导入范围，并确认所选参考文档，以及随后包含的用户指南和语言参考（如果纳入），在 DevDocs 中可用。

由索引模型根据 Issue 内容生成。